1. Field of Invention
The present invention relates to a charge pump. More particularly, the present invention relates to a circuit and a method for improving the efficiency of charge pump circuits and the charge pumps which use the circuit and the method.
2. Description of Related Art
Generally, a traditional charge pump circuit includes a voltage source, one or more charge capacitance, a load capacitance, a number of circuit switches and a fixed-frequency clock used to control a number of circuit switches. Using a clock period as an example (e.g. a doubled two phases circuit), in the first half period, circuit switches are used to make a parallel connection between a voltage source and a charge capacitance so as to charge the charge capacitance to a voltage level; in the second half period, circuit switches are used to make a serial connection between the voltage source and the charge capacitance and a load capacitance. After a number of periods are repeated, the voltage difference between two sides of the load capacitance will be lifted up to a voltage level that is much higher than that of the original voltage source.
FIG. 1A illustrates a traditional charge pump which is controlled by the fixed input frequency clock Ø1 and clock Ø2. FIG. 1B illustrates the equivalent circuits of the traditional circuit shown in FIG. 1A when the clock Ø1 and the clock Ø2 are in high voltage. Referring to FIG. 1B, when clock ψ1 turns the switches SW1 and SW4 on, the input voltage Vi charges the capacitance C1 to the level of voltage source Vi. Here assuming the electric charge stored on the capacitance C2 is zero, when the clock Ø2 turns the switches SW2 and SW3 on, the electric charge stored on the capacitance C1 will be redistributed between the capacitance C1 and the capacitance C2 which is connected with the capacitance C1 in parallel. After repeating the period a number of times, the voltage difference between two sides of the capacitance C2 can be lifted up to twice as much as the input voltage level, viz. 2Vi.
The voltage required for such circuit design can be achieved by using different progressions of the charge pumps in accordance with the needed voltage levels. However, as the consumption of electric charge stored on the load capacitance by the load circuit, the voltage stored on the load capacitance will be reduced along with the consumption of the load. In order to maintain the voltage stored on the load capacitance, after the load capacitance reaches the target voltage level, it is necessary that the switches of the charge pump continuously charge the load capacitance with a fixed frequency. Therefore, the capacitance C1 has to obtain electric charge from the voltage source Vi at a fixed period and supplies the electric charge to the capacitance C2 so as to maintain the voltage of the capacitance C2. This mechanism results in the phenomenon of ripple that the output voltage level of the charge pump has the same clock frequency with the input of the charge pump. The size of the ripple is in inverse ratio to the load capacitance, in direct ratio to the load power consumption and in inverse ratio to the input frequency of the charge pump circuit.
FIG. 2 illustrates the ripple oscillogram produced by the traditional charge pump under the operation of different clock frequencies. As shown in FIG. 2, at the moment of discharging clock Ø1 and charging clock Ø2 will cause the output voltage to occur a ripple whose size is related to the frequency of the supplementary electric charge of the charge pump circuit and the size of the load current. To a fixed current load, the input clock frequency f1 is slower than the input clock frequency f2; accordingly, the discharge time of the first circuit 210 is longer than that of the second circuit 220 so that the amplitude of the ripple of the first circuit 210, Vripple1, is bigger than that of the second circuit 220, Vripple2.
When the load current of the load circuit driven by the charge pump changes along with time, the choices of the needed input clock frequency and the needed load capacitance are usually determined by the biggest load, so as to meet the lowest demand for the ripple size. When the demand of the load is low, if the input clock frequency is the fixed high input frequency, there will be unnecessary power consumption and the efficiency of the charge pump will be reduced. If a relative low frequency is used as the input clock frequency, too big ripple amplitude will be occurred, resulting in an increase of interference to the load circuit. When a relative big capacitance is used, it will lead to a waste of the area cost and an increase of the load of the voltage source.